You could in principle make use of something like the RTG power sources used by some spacecraft.
This would provide electrical energy sufficient to power a small electric motor.

According to the RTG wiki page238Pu is the best candidate for an RTG putting out 0.54 watts per gram and needing 2.5mm lead to absorb all radiation. According to the electric bicycle wiki page power requirements are 200-750 watts. So you'd need at least 400 grams of plutonium to power it and that's only for pedal assist, if you want a full on electric motorcycle according to this company the power requirement is ~40kw, in other words you'll need nearly 75kg of plutonium...

Not to mention expensive! It's reported at $4,000 per gram. That's $300 million dollars total. If the bike itself was a copy of the most expensive bike sold (that I can find with a quick google) it would add nothing more than a blip to the total bill.